The expression occlusion means the closure of the auditory channel which occurs when wearing a hearing aid. A hearing aid or an earpiece of such an acoustic appliance placed in the ear seals the auditory channel from the external environment. In consequence, the hearing-aid wearer perceives his own voice to be much louder and more distorted than normal. This phenomenon is also referred to as the closure effect or occlusion effect. The occlusion effect is perceived as being highly unpleasant, and also makes it harder to perceive complex environmental noises, such as speech.
The occlusion effect occurs because of oscillations in the wall of the auditory channel. These oscillations are transmitted by means of so-called bone conduction from the vocal chords or other sound sources when speaking or chewing. They cause the walls of the soft part of the auditory channel to oscillate, in a similar way to a sound membrane. If, for example, the outer auditory channel is blocked by an earpiece, these oscillations produce a relatively high sound pressure level, since the sound cannot escape outward as in an open ear. The sound pressure may in this case be up to 30 dB higher than normal on the ear drum. The sound pressure increase depends on the frequency. The occlusion effect is particularly evident at lower frequencies below 1 kHz. The speaker's own voice may be amplified by up to 20 dB at these frequencies.
In order to reduce the occlusion effects which occur in a closed auditory channel, occlusion reduction circuits are also already known, in addition to mechanical solutions, for example so-called vent openings. In this case, loop filters are used, and are arranged in a feedback loop of the respective acoustic appliance. The output signal from the loop filter is in this case subtracted from the actual audio signal in order to attenuate the frequencies that have been amplified by the occlusion effect. So-called compensation filters are also used in order to compensate for the distortion caused by the occlusion reduction circuit itself, and are arranged in the transmission path of the audio signal. Both the loop filter and the compensation filter are in this case in the form of static filters, with predetermined coefficients.
However, it has been found that the conditions in which the occlusion reduction circuit operates can vary. This can relate to virtually all components of the acoustic system involved in the signal processing and to all the variables which could influence the signals. For example, the auditory channel may be widened when wearing a hearing aid. In consequence, the transfer function of the corresponding variable also changes. Furthermore, during operation, a hearing aid is also subject to various external influences, such as different noise links which, for example, can influence the audibility of different noise sources. A static system for reduction of occlusion effects is not able to ensure optimum performance and thus comprehensibility in all the various operating conditions.